Heretofore, there has been proposed a positive electrode plate for an alkaline storage battery, which plate comprises a foamed nickel substrate having three-dimensionally continuous pores with a porosity of approximately 95% and an active material, such as spherical nickel hydroxide particles, retained in the substrate. At present, this positive electrode is being widely used as a positive electrode capable of yielding a high-capacity alkaline storage battery. However, the above-mentioned foamed nickel substrate is quite expensive, because it is produced by coating a polyurethane foam with nickel plating, followed by firing to remove the polyurethane foam.
In contrast, core materials having a two-dimensional structure, such as a punched metal sheet or an expanded metal sheet, are inexpensive, since they are generally produced by mechanical processes. Such core materials having no three-dimensional structure, however, may present such problems as detachment (peeling or exfoliation) of the active material, reduction in active material utilization rate, and reduction in high-rate discharge characteristic.
Attempts have been made to process a metal plate into a three-dimensional structure in order to suppress the detachment of the active material and the reduction in active material utilization rate. For example, there has been proposed a nickel positive electrode for an alkaline storage battery, which employs a core material (hereinafter, referred to as “core material X”) having square through-holes with pyramidal projections protruding in opposite directions in an alternating manner (e.g., see Japanese Unexamined Patent Publication No. Hei 7-130370).
However, even the use of the above-described core material X is not sufficient to suppress the detachment of the active material from the electrode plate. More specifically, when the electrode plate is pressed into a predetermined thickness in the manufacturing process of the electrode plate in order to improve the packing density of the active material, a stress is applied to the interface between the core material and the active material layer owing to their difference in rate of elongation, thereby in some cases resulting in the peeling of the active material layer from the core material. Accordingly, the current collection properties become insufficient, making it impossible to yield a sufficient high-rate discharge characteristic. In addition, the expansion and contraction of the active material, due to repeated charge/discharge cycles, also cause the active material layer to be peeled off from the core material, resulting in the problem of decreased capacity.
Meanwhile, attempts have been made to form fine projections comprising a metal powder, such as nickel powder, on the surface of the above-described core material X (e.g., see Japanese Unexamined Patent Publication No. Hei 9-7603). However, even with the use of core material X having the fine projections, it is impossible to solve the above-discussed problems which occur during the pressing of the electrode plate.
Moreover, it is difficult to uniformly form the fine projections comprising a metal powder on the surface of the core material X. Even the application of a metal powder onto the core material surface by spraying or the like cannot achieve a sufficiently uniform formation of the fine projections. An electrode plate employing such core material is susceptible to the peeling of the active material layer from the core material, so that the discharge characteristic and charge/discharge cycle characteristic of the battery are likely to decrease.
There have also been proposed various methods of roughening the core material surface in order to improve the adhesion between the core material and the active material layer. For example, electrolytic deposition, etching, sandblasting, and the like have been tried. However, these methods are also not sufficient to reduce the above-discussed problems which occur during the pressing of the electrode plate.